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Residual soil derived from metasedimentary rockDescriptionMaterial derived from metasedimentary rock which has undergone chemical weathering to a point wherethe original rock texture cannot be discerned. Residual soil corresponds to rock weathering grade VI (seeTable 4, Table 10 and Plate 3 in Geoguide 3).Key diagnostic features :(i) Usually dominated by dark brown, grey, purple or green clayey and oxide-rich material.(ii) The clayey material described in (i) often contains angular fragments of siliceous metasedimentaryrock, ranging from fine sand to cobble size. There are usually no discrete quartz crystals as inresidual soil derived from igneous rocks,(iii)The characteristics, in particular the colour, of residual soil derived from metasedimentary rocksusually vary considerably over short depth intervals, reflecting the variability in character of theparent rocks as well as secondary chemical processes.CommentsResidual soil derived from metasedimentary rock can have characteristics that are very similar to those ofunconsolidated cavity-fill deposits. Careful examination of Mazier samples may help to distinguish betweenthem.Reference sample no.:Reference drillcores C-42 (55 - 94.5 m) and CC-9 (118 - 130 m).FaultDescriptionA discontinuity in the rock containing fault breccia. Definition and description of faults and related featuresare given in Geoguide 3, Section A7.2.Key diagnostic features :(i) Presence of fault breccia.(ii) Fault breccias are unlikely to be wider than 1 metre, and are usually considerably less.CommentsHealed faults are those in which a (usually siliceous) cement has sealed the discontinuity and is contiguouswith the wallrock minerals. A healed fault is no longer a physical discontinuity, and therefore may not besignificantly weaker than the enclosing rock.Reference sample no.: Reference drillcore E-96 (80 - 92 m)Plate A23 - Healed fault and shearing (see Section 2.28) in contact fades granite. Tung Chung, Site 3;Drillcore CC-23; Box 4, 66 m